System and method of driving a display device

ABSTRACT

Electric power consumed in an operation of displaying a gray-scale image is reduced. One of a plurality of scanning lines is selected during one horizontal scanning period, and a selection voltage is applied to the scanning line during one of a first half period and a second half period that said horizontal scanning period has been divided into. When an intermediate gray level is displayed, the selection voltage is applied to a scanning line in an odd-numbered column during the second half period and the selection voltage is applied to a scanning line in an even-numbered column during the first half period. To a pixel at a location corresponding to the selected scanning line, a turn-on voltage and a turn-off voltage are applied via a corresponding data line during particular periods within the period during which the selection voltage is applied such that the turn-on voltage is applied during a period with a length corresponding to a gray level while the turn-off voltage is applied during the remaining period. Thus, when an intermediate gray level is displayed by a pixel, the voltage level of the data signal Xi applied to the pixel is switched as few times as twice per horizontal scanning period, and thus electric power consumed in switching the voltage level is suppressed.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to a method of driving a display devicesuch that a gray-scale image is displayed by using pulse widthmodulation with reduced electrical power consumption. The presentinvention also relates to a driver circuit based on such a method, adisplay device, and an electronic device.

2. Description of Related Art

In general, a portable electronic device includes a display device forpresenting various kinds of information to a user. In such displaydevices, information is displayed using an electrooptical change in anelectrooptical material. For example, liquid crystal display devices arewidely used for this purpose. In recent years, it has become desirablethat display devices be not only capable of simply providing anachromatic display (ON/OFF or two-value black and white), but also becapable of representing a large number of gray levels so that images ofintermediate gray levels can be displayed.

However, in portable electronic devices which are powered by a battery,it is very important that the portable electronic devices operate withsmall power consumption. As is well known, extremely greater powerconsumption is needed to display a gray-scale image relative to thatwhich is needed to display a simple black-and-white image. That is, indisplay devices for use in portable electronic devices, it is necessaryto meet the requirements of having the capability of displaying agray-scale image and having small power consumption, which oftenconflict with each other.

SUMMARY OF THE INVENTION

In view of the above, it is an object of the present invention toprovide a method of driving a display device so as to display agray-scale image without causing a significant increase in powerconsumption, a driver circuit for implementing the method and a displaydevice using such a method, and an electronic device using such adisplay device.

According to a first aspect of the present invention, there is provideda method of driving a display device so as to display a gray-scale imageby driving pixels disposed at locations corresponding to respectiveintersections of a plurality of scanning lines extending along rows anda plurality of data lines extending along columns. The method comprisingthe steps of: selecting a single scanning line from the plurality ofscanning lines during one horizontal scanning period and applying aselection voltage to the scanning line during one of half periods of thehorizontal scanning period; and selecting a single scanning lineadjacent to the previously selected scanning line during following onehorizontal scanning period and applying a selection voltage to theadjacent scanning line during the other one of the half periods of thehorizontal scanning period; while at the same time applying a turn-on orturn-off voltage to a pixel at a location corresponding to a selectedscanning line via a corresponding data line such that the turn-onvoltage is applied during a period with a length corresponding to a graylevel in the period during which the selection voltage is applied andthe turn-off voltage is applied during the remaining period. In thisfirst aspect of the present invention, a reduction is achieved in thenumber of times the voltage applied to a data line is switched betweenthe turn-on voltage and the turn-off voltage during an operation ofdisplaying pixels having intermediate gray levels, and thus it ispossible to reduce electric power consumed in switching the voltage.

In this first aspect of the present invention, in the case where onlywhite or black pixels are displayed and no intermediate gray levels aredisplayed, the number of times the voltage applied to a data line isswitched between the turn-on voltage and the turn-off voltage does notdecrease, and thus an increase in power consumption can occur. To avoidsuch a problem, in this first aspect of the present invention, it isspecified whether or not the mode should be changed. In the case whereit is specified that the mode should be changed, when the adjacentscanning line is selected in the following one horizontal scanningperiod, the selection voltage is preferably applied to the adjacentscanning line during one of half periods of the horizontal scanningperiod. That is, when an intermediate level is not displayed, the modeis changed to prevent an increase in power consumption.

A command to change the mode may be issued by an application, or a user.Alternatively, the gray level data associated with the pixels areexamined, and a command to change the mode may be issued depending upona result of the examination. In this case, it is preferable that themode shift be specified when the number of pixels successively alignedalong a column in which pixels will be displayed in a single color ofeither black or white exceeds a predetermined number of pixels locatedalong a single scanning line that will be selected. This prevents anincrease in power consumption.

Furthermore, in the first aspect of the present invention, it ispreferable that the mode shift be prevented when pixels the number ofpixels successively aligned along a column in which black and whitepixels will be displayed alternately exceeds a predetermined number ofpixels located along a single scanning line that will be selected. Thisis because if the mode is changed in such a case, an increase occurs inthe number of times the voltage applied to the data line of the pixelsis switched between the turn-on voltage and the turn-off voltage, andthus power consumption increases.

According to a second aspect of the present invention, in order toachieve the above-described object, there is provided a driver circuitfor driving a display device so as to display a gray-scale image bydriving pixels disposed at locations corresponding to respectiveintersections of a plurality of scanning lines extending along rows anda plurality of data lines extending along columns. The driver circuitcomprising: a scanning line driver circuit for selecting out of saidplurality of scanning lines a single scanning line during a horizontalscanning period and applying a selection voltage to the scanning lineduring one of two half periods that said horizontal scanning period hasbeen divided into, and for selecting a single scanning line adjacent tothe previously selected scanning line during the subsequent horizontalscanning period and applying a selection voltage to the adjacentscanning line during the other of the two half periods of saidhorizontal scanning period; and a data line driver circuit for applyinga turn-on or turn-off voltage to a pixel at a location corresponding tothe scanning line selected the scanning line driver circuit via acorresponding data line such that the turn-on voltage is applied duringa period with a length corresponding to a gray level in the periodduring which said selection voltage is applied and the turn-off voltageis applied during the remaining period. According to this second aspectof the present invention, as with the first aspect of the presentinvention, a reduction can be achieved in the number of times thevoltage applied to a data line is switched between the turn-on voltageand the turn-off voltage, and thus it is possible to reduce electricpower consumed in switching the voltage.

According to a third aspect of the present invention, to achieve theabove-described object, there is provided a display device fordisplaying a gray-scale image by driving pixels disposed at locationscorresponding to respective intersections of a plurality of scanninglines extending along rows and a plurality of data lines extending alongcolumns. The display device comprising: a scanning line driver circuitfor selecting out of the plurality of scanning lines a single scanningline during a horizontal scanning period and a selection voltage to thescanning line during one of two half periods that the horizontalscanning period has been divided into, and selecting a single scanningline adjacent to the previously selected scanning line during thesubsequent horizontal scanning period and applying a selection voltageto the adjacent scanning line during the other of the two half periodsof the horizontal scanning period; and a data line driver circuit forapplying a turn-on or turn-off voltage to a pixel at a locationcorresponding to the scanning line selected the scanning line drivercircuit via a corresponding data line such that the turn-on voltage isapplied during a period with a length corresponding to a gray level inthe period during which said selection voltage is applied and theturn-off voltage is applied during the remaining period. According tothis third aspect of the present invention, as with the first or thesecond aspect of the present invention, a reduction is achieved in thenumber of times the voltage applied to a data line is switched betweenthe turn-on voltage and the turn-off voltage, and thus it is possible toreduce electric power consumed in switching the voltage.

In this third aspect of the present invention, the pixel preferablyincludes a switching element and a capacitor driven by the switchingelement. In this construction, a selected pixel and a non-selected pixelis electrically isolated from each other by the switching element, andthus good contrast and response can be obtained and a high-quality imagecan be displayed.

In this construction, the switching element is a thin film diode havinga conductor/insulation/conductor structure. In this case, one end of thethin film diode is connected to either a scanning line or a data line,and the other end thereof is connected to the capacitor. When the thinfilm diode is used as the switching element, the production processbecomes simpler. Besides, principally, no short-circuited path iscreated between a scanning line and a data line.

According to a fourth aspect of the present invention, there is providedan electronic device including a display device according to theprevious aspect of the invention. The electronic device according to thepresent invention is capable of displaying a gray-scale image withreduced power consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments of this invention will be described in detail,with reference to the following figures, wherein like reference numeralsreference like elements, and wherein:

FIG. 1 is an exemplary block diagram illustrating the electricalconfiguration of a display device according to an embodiment of thepresent invention;

FIG. 2 is a perspective view illustrating the structure of a liquidcrystal panel of the display device;

FIG. 3 is a fragmentary perspective view illustrating the structure of amain part of the liquid crystal panel;

FIG. 4 is a block diagram illustrating the structure of a Y driver ofthe display device;

FIG. 5 is a timing chart illustrating an operation of the Y driver;

FIG. 6 is a timing chart illustrating an operation of the Y driver;

FIG. 7 is an exemplary block diagram illustrating the structure of an Xdriver of the display device;

FIG. 8 is a timing chart illustrating an operation of the X driver;

FIG. 9 is a timing chart illustrating an operation of the X driver;

FIG. 10 is a timing chart illustrating voltage waveforms of a datasignal Xi for various combinations of gray levels for the case in whicha discrimination signal SG is at a high level;

FIG. 11 is a timing chart illustrating voltage waveforms of the datasignal Xi for various combinations of gray levels for the case in whichthe discrimination signal SG is at a low level;

FIGS. 12(a) and 12(b) are equivalent circuit diagrams of a pixel of adisplay device according to an embodiment;

FIG. 13 is a diagram illustrating examples of waveforms of a scanningsignal Yj and a data signal Xi according to a 4-value driving (invertingevery 1H) method;

FIG. 14 is a diagram illustrating a problem in a displaying operation;

FIG. 15 is a diagram illustrating examples of waveforms of a scanningsignal Yj and a data signal Xi according to a 4-value driving (invertingevery ½H) method;

FIG. 16(a) is a diagram illustrating a right-side modulation method, andFIG. 16(b) is a diagram illustrating a left-side modulation method;

FIGS. 17(a) and 17(b) are diagrams illustrating power consumption inswitching of the voltage of a data signal Xi during a retention period;

FIG. 18 is a diagram illustrating examples of waveforms of a scanningsignal Yj and a data signal Xi according to the right-side modulationmethod;

FIG. 19 is a perspective view illustrating a structure of a personalcomputer which is an example of an electronic device using the displaydevice;

FIG. 20 is a perspective view illustrating a structure of a portabletelephone which is an example of an electronic device using the displaydevice; and

FIG. 21 is a perspective view illustrating a structure of a digitalstill camera which is an example of an electronic device using thedisplay device.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The electrical configuration of a display device according to anembodiment of the present invention is described below. FIG. 1 is anexemplary block diagram illustrating the electrical configuration of thedisplay device. In a liquid crystal panel 100, as shown in FIG. 1, datalines such as segment electrodes 212 are formed so as to extend alongcolumns (in a Y direction), and scanning lines such as common electrodes312 are formed so as to extend along rows (in an X direction). In thepresent embodiment, by way of example but not of limitation, there are atotal of 240 scanning lines 312 and a total of 320 data lines 212, andthe display device is formed so as to serve as a 240×320 matrix displaydevice. Pixels 116 are formed at respective locations corresponding tointersections of the data lines 212 and the scanning lines 312. Eachpixel 116 is composed of a series connection of a liquid crystal layer118 and a TFD (Thin Film Diode) 220 serving as a switching element.

A Y driver 350, which is also called a scanning line driver circuit,serves to supply scanning signal Y1, Y2, . . . , Y240 to correspondingscanning lines 312. More specifically, the Y driver 350 selects thescanning lines 312 one by one and applies a selection voltage to theselected scanning line during one of a first half period and a secondhalf period of the selection period and applies a non-selection voltageto the selected scanning line during the other one of the first halfperiod and the second half period of the selection period.

An X driver 250, which is also called a data line driver circuit, servesto supply data signals X1, X2, . . . , X320, corresponding to a contentto be displayed, to pixels 116 located along a scanning line 312selected by the Y driver 350, via corresponding data lines 212. The Xdriver 250 also outputs a discrimination signal SG to a control circuit400. The discrimination signal SG specifies a mode according to thepresent embodiment, as will be described in greater detail later. Thedetailed structures of the X driver 250 and the Y driver will also bedescribed later.

The control circuit 400 supplies various control signals and a clocksignal to the X driver 250 and the Y driver 350 to control them. Adriving voltage generator 500 generates voltages of ±VD/2 and ±VS,wherein voltages of ±VD/2 are used as data voltages of a data signal andalso as non-selection voltages of a scanning signal, and voltages of ±VSare used as selection voltages of a scanning signal.

In the present embodiment, the polarities of voltages applied to thescanning lines 312 and the data lines 212 are defined such that themiddle potential of the data voltages ±VD/2 applied to the data lines212 is employed as a reference voltage and potentials higher than thereference voltage are regarded as positive and those lower than thereference voltage as negative.

The mechanical structure of the display device according to the presentembodiment is described below. FIG. 2 is a perspective view generallyillustrating the structure of the display device. As shown in FIG. 2,the liquid crystal panel 100 includes a device substrate 200 and anopposite substrate 300 which are adhesively connected to each other. Thedevice substrate 200 has a part extending outward beyond an edge of theopposite substrate 300, wherein the upper surface of this extending partserves as a terminal area. In this terminal area, the X driver 250 inthe form of a bare chip is mounted by means of a COG (Chip On Glass)technique. Furthermore, one end of an FPC (Flexible Printed Circuit)board 260 is connected to the terminal area of the device substrate 200so as to supply various signals to the X driver 250. Similarly, theopposite substrate 300 has a part extending outward beyond an edge ofthe device substrate 200, wherein the lower surface of this extendingpart serves as a terminal area. In this terminal area of the oppositesubstrate 300, the Y driver 350 in the form of a bare chip is mounted bymeans of a COG technique, and one end of an FPC board 360 is connectedto the terminal area of the device substrate 300 so as to supply varioussignals to the Y driver 350. The other ends of the FPC substrates 260and 360 are respectively connected to the control circuit 400 and thedriving voltage generator 500 shown in FIG. 1.

The X driver 250 and the Y driver 350 are mounted as follows. First,they are placed at predetermined locations on the correspondingsubstrate such that anisotropic conductive films formed by uniformlydispersing conductive microparticles into an adhesive material areplaced between the respective chips and the substrates. The bare chipsof the X and Y drivers are pressed against the respective substratewhile heating them. Connecting of the FPC substrates 260 and 360 is alsoperformed in a similar manner. Instead of mounting the X driver 250 andthe Y driver 350 on the device substrate 200 and the opposite substrate300, respectively, they may be mounted on a TCP (Tape Carrier Package)and connected using a TAB (Tape Automated Bonding) technique such thatelectrical and mechanical connections of the X driver 250 and the Ydriver 350 are achieved via an anisotropic conductive film disposed at aparticular location on a substrate.

The detailed structure of pixels 116 of the liquid crystal panel 100 isdescribed below. FIG. 3 is a fragmentary perspective view illustratingsome pixels. As shown in FIG. 3, pixel electrodes 234 formed of atransparent conducting material, such as ITO (Indium Tin Oxide), aredisposed in the form of a matrix on the surface, facing the oppositesubstrate, of the device substrate 200. Of these pixel electrodes 234,240 pixel electrodes 234 arranged in the same single column areconnected via corresponding TFDs 220 to one of data lines 212 extendingin a Y direction. Each TFD 220 is formed so as to have a sandwichstructure of conductor/insulator/conductor using a first conductor 222which is formed of tantalum in the form of elementary substance or antantalum alloy and which extends as a branch from one of the data lines212, an insulator 224 obtained by anodizing the first conductor 222, anda second conductor 226, thus chromium such that the TFD 220 has a diodeswitching characteristic having nonlinearity in current-voltagecharacteristic in both forward and reverse directions.

An insulating film 201, which is transparent and electricallyinsulating, is formed on the upper surface of the device substrate 200.This insulating film 201 serves to prevent the first conductor 222 frombeing peeling off during a heat treatment performed after deposition ofthe second conductor 226 and also serves to prevent an impurity fromdiffusing into the first conductor during the heat treatment. When noproblem associated with the peeling off and the diffusion occurs, theinsulating film 201 may be eliminated.

On the surface, facing the device substrate, of the opposite substrate300, scanning lines 312 formed of ITO or the like extend in a rowdirection perpendicular to the direction in which the data lines 212extends, wherein the scanning lines 312 are disposed at locationscorresponding to the pixel electrodes 234 so that the scanning lines 312serve as opposite electrodes opposing the pixel electrodes 234.

The device substrate 200 and the opposite substrate 300 are spaced apredetermined distance from each other by a sealing material (not shown)coated on the substrates in a peripheral region and also by spacers (notshown) properly distributed. A liquid crystal 105 of, for example, theTN (Twisted Nematic) type is disposed and sealed in a closed spacebetween the device substrate 200 and the opposite substrate 300. Thus,each liquid crystal layer 118 shown in FIG. 1 at a location where a dataline 212 and a scanning line 312 cross each other is formed of thescanning line 312, a pixel electrode 234, and a corresponding part ofthe liquid crystal 105 disposed between the scanning line 312 and thepixel electrode 234.

Furthermore, although not shown in the figure, depending upon anapplication in which the liquid crystal panel 100 is used, color filtersare disposed in the form of stripes, a mosaic, or triangles on theopposite substrate 300. The other areas are covered with a black matrixfor blocking light. Furthermore, alignment films rubbed in particulardirections are disposed on the mutually-facing surfaces of the devicesubstrate 200 and the opposite substrate 300, respectively, andpolarizers or the like corresponding to the alignment directions aredisposed on the back faces of the respective substrates.

One pixel 116 having the above-described structure can be represented byan equivalent circuit, such as that shown in FIG. 12(a). In FIG. 12(a),one pixel 116 is represented by a series circuit of a TFD 220 and aliquid crystal layer 118, wherein the TFD 220 is represented by aparallel circuit of a resistor RT and a capacitor CT and the liquidcrystal layer 118 is represented by a parallel circuit of a resistor RLCand a capacitor CLC.

A data signal Xi and a scanning signal Yj are applied, by means of apredetermined driving method, to respective two ends of the pixel 116represented by the above equivalent circuit. Herein, the data signal Xiis assumed to be a data signal applied to a data line 212 of the ithcolumn as counted from the leftmost column in FIG. 1, and the scanningsignal Yj is assumed to be a scanning signal applied to a scanning line312 of thejth row as counted from the top row in FIG. 1.

A 4-value driving method (inverting every 1H) is widely used as adriving method. FIG. 13 illustrates examples of waveforms of thescanning signal Yj and the data signal Xi which are applied to a certainpixel 116 in accordance with the 4-value driving method. In this drivingmethod, a selection voltage +VS is first applied as the scanning signalYj during one horizontal scanning period 1H, and then, during afollowing retention period, a non-selection voltage +VD/2 is applied. Ifone vertical scanning period (one frame period) 1V has elapsed since theprevious selection, a selection voltage −VS is applied. In a followingretention period, a non-selection voltage −VD/2 is applied. Whileperforming the above operation repeatedly, either a data voltage +VD/2or −VD/2 is applied as the data signal Xi. If a selection voltage +VS isapplied as the scanning signal Yj to a certain scanning line, aselection voltage −VS is applied as the scanning signal Yj+1 to thesubsequent scanning line. In this 4-value driving method (invertingevery 1H), in the case +VS is applied as the selection voltage +VS,where a pixel 116 is turned on (to provide a black display ), −VD/2 isapplied as the data signal. On the other hand, to turn off the pixel 116(to provide a white display), +VD/2 is applied as the data signal. Inthe case where −VS is applied as the selection voltage, when black isdisplayed by a pixel 116, +VD/2 is applied as the data signal, while−VD/2 is applied when white is displayed by the pixel 116.

In this 4-value driving method (inverting every 1H), if a pattern isdisplayed in a partial area A of a screen 100 a such that black andwhite are alternately displayed from one scanning line to next as shownin FIG. 14, crosstalk occurs in this area A in the Y direction.

The reason is briefly described below. When the above pattern isdisplayed in the area A, the data signals applied to the data line inthis area are periodically switched between ±VD/2 at the same intervalsas the scanning signals are inverted, and thus the voltages of the datasignals are fixed to either +VD/2 or −VD/2 over a period during whichselection lines in the area A are selected. When viewed along pixels inthe Y direction in the area A, the data voltages are fixed to either+VD/2 or −VD/2 during a particular part of a retention period. On theother hand, as described earlier, the selection voltages applied toadjacent scanning lines are opposite in polarity. Therefore, in areasadjacent to the area A in the Y direction, differences occur in theeffective values of the voltage applied during the part of the retentionperiod between pixels 116 in odd-numbered rows and those ineven-numbered rows. As a result, in the areas adjacent to the area A inthe Y direction, differences occur in the gray levels between the pixels116 in odd-numbered rows and those in even-numbered rows, and thuscrosstalk occurs.

One technique of preventing the above problem is to employ a 4-valuedriving (inverting every ½H) method. Each horizontal scanning period 1Hin the 4-value driving (inverting every 1H) method is divided into afirst and second half periods. A scanning line is selected, for example,during each first half period (½H), and data voltages −VD/2 and +VD/2are respectively applied during periods of 50% of the one totalhorizontal scanning period 1H. According to the 4-value driving(inverting every ½H) method, even when any pattern is displayed, thedata signal Xi has a voltage equal to −VD/2 during a half period of ahorizontal scanning period and has a voltage equal to +VD/2 during theother half period, and thus no crosstalk occurs.

A driving method for obtaining a gray-scale image is described below.Two known methods of displaying a gray-scale image are voltagemodulation and pulse width modulation. In the voltage modulation,controlling a voltage so as to obtain a desired gray level is difficult.For this reason, the pulse width modulation is more widely used. Thepulse width modulation may be applied to the 4-value driving (invertingevery ½H) method in three different manners. In a first manner calledright-side modulation, a turn-on voltage is applied during a periodimmediately before the end of a selection period as shown in FIG. 16(a).

In a second manner called left-side modulation, a turn-on voltage isapplied during a period at the start of a selection period as shown inFIG. 16(b).

In a third manner called distributed modulation (not shown), turn-onvoltages with time widths corresponding to weights of respective bits ofgray-level data are distributed during a selection period. Herein, theturn-on voltage refers to a voltage of a data signal Xi which is appliedto a data line 212 of ith column to write data into a pixel 116, whereinthe turn-on voltage has an opposite polarity to that of a selectionvoltage ±V<SUB>S</SUB>[±VS] during a period in which the selectionvoltage is applied.

Of the three modulation methods, the left-side modulation and thedistributed modulation have a problem that discharging occurs after aturn-on voltage is written, and thus it is difficult to obtain precisegray levels. Besides, a high driving voltage is needed in these methods.For the above reason, when a gray-scale image is displayed using the4-value driving method, the right-side modulation is usually employed.Therefore, it is assumed that the right-side modulation is used in thefollowing description, although the present invention may also beapplied to the left-side modulation.

In the display device shown in FIG. 1, because there are a total of 240scanning lines 312, a retention period (non-selection period) in onevertical scanning period 1V is equal to 239 times one horizontalscanning period 1H, that is, 239H. In each retention period, the TFD 220is turned off, and thus the resistance RT of the TFD 220 becomes verylarge. On the other hand, the resistance RLC of the liquid crystal layer118 is very large regardless of whether TFD 220 is in an on-state oroff-state. Therefore, in the retention period, the equivalent circuit ofthe pixel 116 can be represented by a capacitor Cpix equivalent to aseries of capacitors CT and CLC as shown in FIG. 12(b). Herein, thecapacitance Cpix is equal to (CT·CLC)/(CT+CLC).

When a certain scanning line 312 is in a non-selected state and anon-selection signal having a voltage of +VD/2 is applied as thescanning signal Yj to that scanning line 312, the data voltage of thedata signal Xi is alternately switched to +VD/2 or −VD/2, as shown inFIG. 17(a) or 17(b). Although not shown in the figure, when anon-selection voltage having a voltage of −VD/2 is applied as thescanning signal Yj to that scanning line, the data voltage of the datasignal Xi is also switched alternately to +VD/2 or −VD/2. Therefore, inone pixel 116, even in a retention (non-selection) period, a chargeequal to Cpix·VD is supplied from a power supply when the voltage of thedata signal Xi is switched twice, and thus power is consumed by acapacitive load of the pixel 116.

In the case where the right-side modulation is used to display agray-scale image using the 4-value driving method, when pixels 116 inone column corresponding to a certain data line 212 are white (off) orblack (on), the voltage of the data signal Xi for this data line 212 isswitched once during one horizontal period 1H as shown in FIG. 18.However, when intermediate gray levels (for example, near white or nearblack ) are displayed at pixels 116 in a certain column, the voltage ofthe data signal Xi for this column is switched three times during onehorizontal period 1H as shown in FIG. 16. Therefore, when a certainpixel 116 is at an intermediate gray level, electric power consumed inthe retention period becomes three times greater than that consumed whenthe pixel 116 is white or black .

In the display device according to the present embodiment of theinvention, to prevent the above problem, as shown in FIG. 5, either of+V<SUB>S</SUB> or −V<SUB>S</SUB>[+VS or −VS] is applied as the selectionvoltage to a scanning line 312 in an odd-numbered row during the secondhalf period of one horizontal scanning period, while either of+V<SUB>S</SUB> or −V<SUB>S</SUB>[+VS or −VS] is applied as the selectionvoltage to a scanning line 312 in an even-numbered row during the firsthalf period of one horizontal scanning period, so that the voltage ofthe data signal Xi applied to a pixel to display an intermediate graylevel is switched twice during one horizontal period 1H as shown in FIG.8 or FIG. 10(c) thereby suppressing power consumption during theretention period. A circuit used to perform such a driving operation isdescribed below.

First, various control signals such a clock signal and other controlsignals generated by the control circuit 400 are described. A startpulse YD is generated at the beginning of each vertical scanning period(each frame period) as shown in FIG. 5 or 6. A clock signal YCLK is areference signal associated with the scanning lines. As shown in FIG. 5or 6, the clock signal YCLK has a period equal to one horizontalscanning period 1H.

An AC driving signal MY is a signal for controlling the pixels 116 to bedriven via the scanning lines in an AC fashion. As shown in FIG. 5 or 6,the signal level of the AC driving signal MY is inverted everyhorizontal scanning period 1H, and furthermore, during the horizontalscanning period in which the same scanning line is selected, the signallevel associated with the same scanning line is inverted every verticalscanning period.

Either one of control signals INHa and INHb is used in an exclusivefashion depending upon the level of the discrimination signal SG tospecify a period in a horizontal scanning period during which aselection voltage is to be applied. The control signal INHa is used whenthe level of the discrimination signal SG is high.

As shown in FIG. 5, the control signal NHa has a period twice the periodof the clock signal YCLK, and becomes high in the second half period ½Hof a horizontal scanning period during which a scanning line 312 in anodd-numbered row is selected and also in the first half period ½H of ahorizontal scanning period during which a scanning line 312 in aneven-numbered row is selected. On the other hand, the control signalINHa is used when the level of the discrimination signal SG is low.

As shown in FIG. 6, the control signal NHb has a period equal to theperiod of the clock signal YCLK, and becomes high in the second halfperiod ½H of a horizontal scanning period during which a scanning line312 in an odd-numbered row is selected and also in the second halfperiod ½H of a horizontal scanning period during which a scanning line312 in an even-numbered row is selected.

A latch pulse LP is generated to latch a data signal on the data lineside. As shown in FIG. 8 or 9, a latch pulse LP is generated at thebeginning of each horizontal scanning period 1H. A reset signal RES isgenerated at the beginning of a first half period and at the beginningof a second half period of each horizontal scanning period 1H as shownin FIG. 8 or 9. As shown in FIG. 8 or 9, an odd/even signal SS becomeshigh during a horizontal scanning period during which a scanning line312 in an odd-numbered row is selected, while the odd/even signal SSbecomes low during a horizontal scanning period during which a scanningline 312 in an even-numbered row is selected. An AC driving signal MX isa signal for controlling the pixels 116 to be driven via the data linesin an AC fashion. As shown in FIG. 8 or 9, the AC driving signal MX ismaintained at an equal signal level during the second half of eachhorizontal scanning period 1H and the first half of the immediatelyfollowing horizontal scanning period 1H. At the end of the first half ofeach horizontal scanning period, the signal level is inverted. The ACdriving signals MX and MY are opposite in polarity to each other duringthe second half of each horizontal scanning period.

Gray level code pulses GCP are placed, as shown in FIG. 8 or 9,immediately before the end of each of the first and second half periodsof each horizontal scanning period 1H, such that the pulse positionsrelative to the end of each half period correspond to the intermediategray level. In the present embodiment, if gray level data for specifyingthe intensity of a pixel is represented by 3 bits so as to indicate oneof 8 gray levels, and if (000) indicates white (off) and (111) indicatesblack (on), gray level code pulses GCP are placed in each of the firstand second half periods such that six pulses corresponding tointermediate gray levels (001) to (110) other than black and white areplaced. More specifically, in FIG. 8 or 9, (001), (010), (011), (100),(101), and (110) of gray level data corresponds to “1”, “2”, “3”, “4”,“5”, and “6” of gray level code pulses. Although in FIGS. 8 and 9 thegray level code pulses GCP are placed at equal intervals for simplicityof illustration, the intervals generally vary depending upon thevoltage-intensity (V-I) characteristic of pixels.

The details of the scanning line driver circuit 350 are described. FIG.4 is an exemplary block diagram illustrating the structure of thescanning line driver circuit 350. In FIG. 4, a shift register 3502 is a240-bit shift register having a register size corresponding to the totalnumber of scanning lines 312. A start pulse YD is supplied to the shiftregister 3502 at the beginning of each frame. The shift register 3502shifts the received start pulse YD in response to a clock signal YCLKhaving the same period as the horizontal scanning period 1H, and outputstransferred signals YS1, YS2, . . . , YS240 such that one signal isoutput at a time. The transferred signals YS1, YS2, . . . , YS240correspond in a one-to-one fashion to the 1st, 2nd, . . . , and 240thscanning lines 312, respectively. When a transferred signal is at a highlevel, a corresponding scanning line 312 is selected.

A voltage selection signal generator 3504 generates, from the AC drivingsignal MY and the control signal INHa or INHb, a voltage selectionsignal which determines a voltage to be applied to a scanning line 312.In the present embodiment, as described earlier, the voltage of thescanning signal to be applied to a scanning line 312 is equal to one offour values: +VS (positive selection voltage), +VD/2 (positivenon-selection voltage), −VS (negative non-selection voltage), and −VD/2(negative selection voltage). Of these voltages, the selection voltage+VS or −VS is applied during the first or second half period ½H of ahorizontal scanning period. If a selection voltage +VS is applied in thecertain second half of a certain horizontal scanning period, thefollowing non-selection voltage has a level equal to +VD/2. Conversely,when −VS is applied as a selection voltage, the following non-selectionvoltage has a level equal to −VD/2. That is, the level of thenon-selection voltage is uniquely determined by the previous selectionvoltage.

To this end, the voltage selection signal generator 3504 generates 240voltage selection signals such that the voltage levels of the scanningsignals Y1, Y2, . . . , Y240 satisfy the following conditions. That is,when one of transferred signals YS1, YS2, . . . , YS240 becomes high anda corresponding scanning line 312 is selected, the voltage selectionsignal generator 3504 generates voltage selection signals so that thevoltage level of the scanning signal applied to that scanning line 312becomes equal to a selection voltage corresponding to the AC drivingsignal MY during a period in which the control signal INHa or INHb is ata high level, and so that in response to a high-to-low transition of thecontrol signal INHa or INHb, the voltage level of the scanning signal ischanged to a non-selection voltage determined depending upon theprevious selection voltage. More specifically, if the AC driving signalMY is high during a period in which the control signal INHa or INHb isactive, the voltage selection signal generator 3504 outputs a voltageselection signal which causes the positive selection voltage +VS to beselected and then outputs a voltage selection signal which causes thepositive non-selection voltage +VD/2 to be selected. On the other hand,if the AC driving signal MY is low during a period in which the controlsignal INHa or INHb is active, the voltage selection signal generator3504 outputs a voltage selection signal which causes the negativeselection voltage −VS to be selected and then outputs a voltageselection signal which causes the negative non-selection voltage −VD/2to be selected. The voltage selection signal generator 3504 generates avoltage selection signal in a similar manner for each of 240 signallines 312.

A level shifter 3506 expands the amplitude of the voltage of the voltageselection signal output from the voltage selection signal generator3504. A selector 3508 selects a voltage specified by the voltageselection signal having an expanded amplitude and supplies the selectedvoltage to a corresponding scanning line 312.

The waveform of the scanning signal supplied from the scanning linedriver circuit 350 having the above structure is described below withreference to FIGS. 5 and 6. As described above, one of the controlsignals INHa and INHb is output depending upon the level of thediscrimination signal SG. First, the operation is described below forthe case where the discrimination signal SG is high and the controlsignal INHa is output.

In this case, as shown in FIG. 5, when a start pulse YD is supplied atthe beginning of a vertical scanning period (one frame period), thestart pulse YD is shifted every horizontal scanning period 1H inresponse to the clock signal YCLK, and the resultant signal is output astransferred signals YS1, YS2, . . . , YS240 one by one. When the controlsignal INHa is active, scanning lines 312 in the odd-numbered rows areselected during the second half period ½H of one horizontal scanningperiod and scanning lines 312 in the even-numbered rows are selectedduring the first half period ½H of one horizontal scanning period,wherein the polarity of the selection voltage is determined dependingupon the level of the AC driving signal MY during the half period ½Hduring which a scanning line of interest is selected.

Thus, the voltage of the scanning signal supplied to a scanning line 312in an odd-numbered row becomes equal to the positive selection voltage+VS if the AC driving signal MY is high during the second half of thehorizontal scanning period and thereafter is changed to the positivenon-selection voltage +VD/2 in response to the polarity of the selectionvoltage and maintained at that voltage. In the second half ½H of ahorizontal scanning period one frame after that, the AC driving signalMY is inverted to a low level, and thus the voltage of the scanningsignal supplied to that scanning line becomes equal to the negativeselection voltage −VS. After the end of that second half period, thevoltage of the scanning signal is changed to the negative non-selectionvoltage −VD/2 in response to the polarity of the previous selectionvoltage and is maintained at that level. For example, the scanningsignal Y1 supplied to a scanning line in the first row as counted fromthe top has a voltage equal to the positive selection voltage +VS duringthe second half of the first horizontal scanning period of an nth frame.At the end of that second half period, the voltage is changed to thenon-selection voltage +VD/2 and maintain at that level. At the beginningof the second half of the first horizontal period of the following(n-1)th frame, the voltage of the scanning signal is changed to thenegative selection voltage −VS. At the end of that second half period,the voltage of the scanning signal is changed to the negativenon-selection voltage −VD/2 and maintained at that level. Thereafter,the above cycle is repeated.

Thus, the voltage of the scanning signal supplied to scanning lines 312in even-numbered rows becomes equal to the negative selection voltage−VS during the second half period ½H of one horizontal scanning periodif the AC driving signal MY is low and thereafter is changed to thenegative non-selection voltage −VD/2 in response to the polarity of theselection voltage and maintained at that voltage. In the first halfperiod ½H of a horizontal scanning period one frame after that, the ACdriving signal MY is inverted to a high level, and thus the voltage ofthe scanning signal supplied to that scanning line becomes equal to thepositive selection voltage +VS. After the end of that first half period,the voltage of the scanning signal is changed to the positivenon-selection voltage +VD/2 in response to the polarity of the previousselection voltage and is maintained at that level. For example, thescanning signal Y1 supplied to a scanning line in the second row ascounted from the top has a voltage equal to the negative selectionvoltage −VS during the first half of the second horizontal scanningperiod of the nth frame. At the end of that first half period, thevoltage is changed to the non-selection voltage −VD/2 and maintain atthat level. At the beginning of the first half period of the secondhorizontal period of the following (n+1)th frame, the voltage of thescanning signal is changed to the positive selection voltage +VS andmaintained at that level. Thereafter, the above cycle is repeated.

Because the signal level of the AC driving signal MY is inverted everyhorizontal scanning period 1H, the voltages of the scanning signalsapplied to adjacent scanning lines become opposite in polarity to eachother and are inverted every horizontal scanning period 1H. For example,as shown in FIG. 5, if the scanning signal Y1 applied to the scanningline which is first selected in the nth frame has a voltage equal to thepositive selection voltage +VS during the second half of that horizontalscanning period, the scanning voltage Y2 applied to a scanning linewhich is selected thereafter has a voltage equal to the negativeselection voltage −VS during the second half of the horizontal scanningperiod during which that scanning line is selected.

Now, an exemplary operation is described below for the case where thediscrimination signal SG is low and the control signal INHb is supplied.As shown in FIG. 6, the control signal INHb is at an active high levelduring the second half period ½H of one horizontal scanning periodregardless of whether a selected scanning line 312 is in an odd-numberedor even-numbered row. Therefore, when any scanning line 312 is selected,it is selected during the second half period ½H of one horizontalscanning period, wherein the polarity of the selection voltage isdetermined depending upon the level of the AC driving signal MY duringthat second half period ½H.

Thus, the voltage of the scanning signal supplied to a scanning line 312in an odd-numbered row becomes equal to the positive selection voltage+VS if the AC driving signal MY is high during the second half of thehorizontal scanning period and thereafter is changed to the positivenon-selection voltage +VD/2 in response to the polarity of the selectionvoltage and maintained at that voltage. In the second half ½H of ahorizontal scanning period one frame after that, the AC driving signalMY is inverted to a low level, and thus the voltage of the scanningsignal supplied to that scanning line becomes equal to the negativeselection voltage −VS. After the end of that second half period, thevoltage of the scanning signal is changed to the negative non-selectionvoltage −VD/2 in response to the polarity of the previous selectionvoltage and is maintained at that level.

The operation performed when the discrimination signal SG is low issimilar to that performed when the discrimination signal SG is high, inthat the scanning signal Y1 supplied to a scanning line in, for example,the first row as counted from the top has a voltage equal to thepositive selection voltage +VS during the second half of the firsthorizontal scanning period of an nth frame, and at the end of thatsecond half period, the voltage is changed to the non-selection voltage+VD/2and maintain at that level. Furthermore, at the beginning of thesecond half of the first horizontal period of the following (n+1)thframe, the voltage of the scanning signal is changed to the negativeselection voltage −VS. Then at the end of that second half period, thevoltage of the scanning signal is changed to the negative non-selectionvoltage −VD/2 and maintained at that level, and thereafter, the abovecycle is repeated.

However, it is different in that the scanning signal Y2 supplied to ascanning line in the second row as counted from the top has a voltageequal to the positive selection voltage +VS during the second half ofthe first horizontal scanning period of the nth frame, and at the end ofthat second half period, the voltage is changed to the non-selectionvoltage +VD/2 and maintain at that level, and furthermore, at thebeginning of the second half of the first horizontal period of thefollowing (n+1)th frame, the voltage of the scanning signal is changedto the negative selection voltage −VS, and then at the end of thatsecond half period, the voltage of the scanning signal is changed to thenegative non-selection voltage −VD/2 and maintained at that level, andthereafter, the above cycle is repeated.

In other words, for the scanning lines 312 in the odd-numbered rows, aselection voltage is applied during the second half period ½H of onehorizontal scanning period regardless of the level of the discriminationsignal SG. However, for the scanning lines 312 in the even-numberedrows, a selection voltage is applied during the first half period ½H ofone horizontal scanning period when the discrimination signal SG ishigh, and a selection voltage is applied during the second half period½H of one horizontal scanning period when the discrimination signal SGis low.

The details of the data line driver circuit 250 are described below.FIG. 7 is a block diagram illustrating the structure of the data linedriver circuit 250. In FIG. 7, an address control circuit 2502 generatesa row address Rad used in reading of gray level data. In response to astart pulse YD supplied at the beginning of a frame, the address controlcircuit 2502 resets the row address Rad. Thereafter, the address controlcircuit 2502 increments the row address Rad in response to a latch pulseLP supplied every horizontal scanning period.

A display data RAM 2504 is a dual port RAM having a memory areacorresponding to 240×320 pixels. On a writing side thereof, gray data Dnsupplied from a processing circuit (not shown) is written at an addressspecified by a write address Wad. On a reading side, one line of graylevel data for 320 pixels at an address specified by a row address Radis read at the same time.

A gray level discriminating circuit 2505 prefetches gray level data forseveral lines stored at addresses preceding the address specified by rowaddress Rad and, when a data signal is generated from the gray leveldata for one line prefetched by the row address Rad, generates adiscrimination signal SG to specify which mode is to be used. Herein, inthe present embodiment, the mode refers to the following operationmodes. That is, in one mode, the scanning signals Y1, Y2, . . . , Y240are generated so as to have a scanning pattern as shown in FIG. 5. Inthe other mode, the scanning signals Y1, Y2, . . . , Y240 are generatedso as to have a scanning pattern as shown in FIG. 6. The criterionemployed in determining the mode depending upon the prefetched graylevel data will be described later.

A PWM decoder 2506 generates voltage selection signals used to selectvoltages of data signals X1, X2, . . . , X320 depending upon the 320gray level data Dn, in accordance with a reset signal RES, an odd/evensignal SS, an AC driving signal MX, a gray level code pulse GCP, and thediscrimination signal SG.

In the present embodiment, the data voltages applied to the data lines212 are equal to either +VD/2 or −VD/2. On the other hand, each graylevel data Dn is represented by 3 bits (8 gray levels). Furthermore, asdescribed above, for scanning lines 312 in odd-numbered rows, aselection voltage is applied during the second half period ½H of onehorizontal scanning period regardless of the level of the discriminationsignal SG, and, for scanning lines 312 in even-numbered rows, aselection voltage is applied during the first or second half period ½Hdepending upon the level of the discrimination signal SG.

Therefore, during a period in which the odd/even signal SS is high (onehorizontal scanning period 1H during which a scanning line 312 in anodd-numbered row is selected), the PWM decoder 2506 generates a voltageselection signal regardless of the level of the discrimination signalSG. However, during a period in which the odd/even signal SS is low (onehorizontal scanning period 1H during which a scanning line 312 in aneven-numbered row is selected), the PWM decoder 2506 generates a voltageselection signal depending upon the level of the discrimination signalSG.

More specifically, for a period during which the odd/even signal SS ishigh, the PWM decoder 2506 generates a voltage selection signalregardless of the level of the discrimination signal SG so that thelevel of a data signal corresponding to one gray level data Dn becomesopposite to the level of the AC driving signal MX in response to a resetsignal RES supplied at the beginning of the first half period ½H of onehorizontal scanning period. Further, the data signal becomes equal tothe level of the AC driving signal MX at a fall [high-to-low transition]of a gray level code pulse GCP corresponding to the gray level data Dn.Accordingly, a reset signal RES supplied at the beginning of the secondhalf period ½H of the horizontal scanning period is neglected and thelevel of the data signal becomes equal to the level of the AC drivingsignal MX at a fall [high-to-low transition] of a gray level code pulseGCP corresponding to the gray level data Dn. However, when a gray leveldata Dn has a value (000) during a period in which the odd/even signalSS is at the high level, the PWM decoder 2506 generates a voltageselection signal so that the corresponding data signal has an oppositelevel to the level of the AC driving signal MX. In the case where a graylevel data Dn has a value (111), the PWM decoder 2506 generates avoltage selection signal so that the corresponding data signal has thesame level as the level of the AC driving signal MX.

On the other hand, for a period during which the odd/even signal SS islow, the PWM decoder 2506 generates a voltage selection signal asfollows depending upon the level of the discrimination signal SG. Thatis, if the discrimination signal SG is high, the PWM decoder 2506generates a voltage selection signal so that the level of a data signalcorresponding to a gray level data Dn becomes the same as the level ofthe AC driving signal MX in response to a reset signal RES supplied atthe beginning of the first half period ½H of one horizontal scanningperiod. Further, the signal becomes opposite to the level of the ACdriving signal MX at a fall [high-to-low transition] of a gray levelcode pulse GCP corresponding to the gray level data Dn. Accordingly, areset signal RES supplied at the beginning of the second half period ½Hof the horizontal scanning period is neglected and the level of the datasignal becomes opposite to the level of the AC driving signal MX at afall [high-to-low transition] of a gray level code pulse GCPcorresponding to the gray level data Dn. However, when a gray level dataDn has a value (000) during a period in which the odd/even signal SS isat the low level and the discrimination signal SG is at the high level,the PWM decoder 2506 generates a voltage selection signal so that thecorresponding data signal has the same level as the level of the ACdriving signal MX. In the case where a gray level data Dn has a value(111), the PWM decoder 2506 generates a voltage selection signal so thatthe corresponding data signal has an opposite level to the level of theAC driving signal MX.

On the other hand, for a period during which the odd/even signal SS islow, if the discrimination signal SG is low, the PWM decoder 2506generates a voltage selection signal so that the level of a data signalcorresponding to a gray level data Dn becomes opposite to the level ofthe AC driving signal MX in response to a reset signal RES supplied atthe beginning of the first half period ½H of one horizontal scanningperiod. The data signal becomes the same as the level of the AC drivingsignal MX at a high-to-low transition of a gray level code pulse GCPcorresponding to the gray level data Dn. Accordingly, a reset signal RESsupplied at the beginning of the second half period ½H of the horizontalscanning period is neglected and the level of the data signal becomesthe same as the level of the AC driving signal MX at a fall [high-to-lowtransition] of a gray level code pulse GCP corresponding to the graylevel data Dn. However, when a gray level data Dn has a value (000)during a period in which the odd/even signal SS is at the high level,the PWM decoder 2506 generates a voltage selection signal so that thecorresponding data signal has an opposite level to the level of the ACdriving signal MX. In the case where a gray level data Dn has a value(111), the PWM decoder 2506 generates a voltage selection signal so thatthe corresponding data signal has the same level as the level of the ACdriving signal MX.

The PWM decoder 2506 generates voltage selection signals for the 320respective gray level data Dn. A selector 2508 selects a voltagespecified by the voltage selection signal generated by the PWM decoder2506 and supplies the selected voltage to a corresponding data line 212one by one.

Thus, the data signal Xi supplied from the data line driver circuit 250has a voltage waveform such as that shown in FIG. 8 when thediscrimination signal SG is high and such as that shown in FIG. 9 whenthe discrimination signal SG is low, wherein, in FIGS. 8 and 9, graylevel data Dn input to the PWM decoder 2506 is represented in binarynumbers and data signals Xi decoded from the gray level data Dn areshown.

The data signal Xi can have various voltage waveforms depending upon thecontent to be displayed at a pixel and depending upon whether thediscrimination signal SG is high or low, as described below. FIG. 10illustrates voltage waveforms the data signal Xi can have when thediscrimination signal SG is high. FIG. 11 illustrates voltage waveformsthe data signal Xi can have when the discrimination signal SG is low. Inboth figures, there are shown waveforms of the data signal Xi for casesin which colors displayed by four pixels 116 at successive locationsalong a column are (a) white-white-white-white, (b)black-black-black-black, (c) gray-gray-gray-gray, (d)white-black-white-black, (e) black-white-black-white, (f)gray-white-gray-white, (g) white-gray-white-gray, (h)gray-black-gray-black, and (i) black-gray-black-gray. Herein, the term“gray” is used to generically describe intermediate gray levels otherthan black and white . More specifically, in the present embodiments,gray levels include those corresponding to gray level data (001), (010),(011), (100), (101), and (110).

As can be seen from FIGS. 10 and 11, in the case where the colorsdisplayed by four pixels 116 at successive locations along a column areone of following combinations, (f) gray-white-gray-white, (g)white-gray-white-gray, (h) gray-black-gray-black, and (i)black-gray-black-gray, the voltage level of the data signal Xi isswitched twice during one horizontal scanning period 1H regardless ofthe level of the discrimination signal SG. However, in the case wherethe colors displayed by four pixels 116 at successive locations along acolumn are one of following combinations, (c) gray-gray-gray-gray, (d)white-black-white-black, and (e) black-white-black-white, when thediscrimination signal SG is high, the voltage level of the data signalXi is switched one time fewer in each horizontal scanning period thanwhen the discrimination signal SG is low. On the other hand, in the casewhere the colors displayed by four pixels 116 at successive locationsalong a column are one of combinations (a) white-white-white-white and(b) black-black-black-black, when the discrimination signal SG is high,the voltage level of the data signal Xi is switched one time more ineach horizontal scanning period than when the discrimination signal SGis low.

Because the electric power consumption decreases with decreasing numberof times the voltage of the data signal Xi is switched (per unit time)as described earlier, the gray level discrimination circuit 2505determines the level of the discrimination signal SG in the mannerdescribed below thereby specifying the mode. That is, when theprefetched gray level data Dn of pixels 116 include a data representinggray, the gray level discrimination circuit 2505 basically sets thediscrimination signal SG to a high level during a horizontal scanningperiod in which the pixel which is to display gray is selected (whereinthe setting of the discrimination signal SG is performed at thebeginning of that horizontal scanning period). However, in a givenparticular horizontal scanning period, if the number of successivepixels displaying gray is smaller than the number of successive pixelsdisplaying white or black, the gray level discrimination circuit 2505sets the discrimination signal SG to have a low level during thathorizontal scanning period.

That is, if the 320 pixels 116 located along a given scanning line 312include pixels which have successive neighboring gray pixels along acolumn, the discrimination signal SG is set to a high level so that thenumber of times the voltage of the data signal is switched perhorizontal scanning period decreases from 3 to 2 thereby achieving areduction in the power consumption. However, when there are pixelshaving successive neighboring gray pixels along a column and pixelshaving successive neighboring white or black pixels and if the number oflatter pixels is greater than the number of former pixels, the goodeffect obtained by reducing the number of times from 3 to 2 for theformer pixels is smaller than the bad effect resulting from the increasein the number of times from 1 to 2 for the latter pixels, and thus theoverall effect is an increase in the power consumption. Therefore, insuch a case, the discrimination signal is set to a low level to avoidthe increase in the power consumption.

In the case where the prefetched gray level data Dn of pixels 116include no gray data (that is, the prefetched gray level data Dn includeonly white or black data) the gray level discrimination circuit 2505basically sets the discrimination signal SG to have a low level during ahorizontal scanning period in which those pixels are selected (whereinthe setting of the discrimination signal SG is performed at thebeginning of that horizontal scanning period). However, in a particularhorizontal scanning period of interest, the number of pixels havingsuccessive neighboring pixels along a column alternately having blackand white pixels is equal to or greater than one-half the total numberof pixels along the scanning line, the gray level discrimination circuit2505 sets the discrimination signal SG to have a high level during thathorizontal scanning period.

That is, if, of 320 pixels 116 located along a scanning line 312 ofinterest, the number of pixels having successive neighboring white orblack pixels along a column is greater than the number of pixels havingsuccessive neighboring pixels along a column alternately having blackand white pixels, the good effect obtained by reducing the number oftimes from 2 to 1 for the former pixels is greater than the bad effectresulting from the increase in the number of times from 1 to 2 for thelatter pixels, and thus the overall effect is an reduction in the powerconsumption. However, if the number of pixels having successiveneighboring white or black pixels along a column is smaller than thenumber of pixels having successive neighboring pixels along a columnalternately having black and white pixels, the good effect obtained byreducing the number of times from 2 to 1 for the former pixels issmaller than the bad effect resulting from the increase in the number oftimes from 1 to 2 for the latter pixels, and thus the overall effect isan increase in the power consumption. Therefore, in such a case, thediscrimination signal is set to a high level to avoid the increase inthe power consumption.

In the display device according to the present embodiment, as describedabove, when gray levels are displayed (by pixels) the discriminationsignal SG is basically set to a high level, while the discriminationsignal SG is basically set to a low level when gray levels are notdisplayed (that is, white or black is displayed by pixels), therebyreducing the number of times the voltage of the data signal is switchedand thus achieving a reduction in power consumption.

However, even when gray levels are displayed, if the setting of thediscrimination signal SG to a high level exceptionally causes anincrease in power consumption, the discrimination signal SG is set to alow level. Conversely, when no gray levels are displayed, if the settingof the discrimination signal SG to a low level exceptionally causes anincrease in power consumption, the discrimination signal SG is set to ahigh level. Thus, an exceptional increase in power consumption isprevented.

In the embodiment described above, the selection period during which aselection voltage is applied to a scanning line 312 in an odd-numberedrow is fixed to the second half period of a horizontal scanning period,while the first or second half period of a horizontal scanning period isemployed, depending upon the level of the discrimination signal, as theselection period during which a selection voltage is applied to ascanning line 312 in an even-numbered row. Alternatively, the selectionperiod during which a selection voltage is applied to a scanning line312 in an odd-numbered row may be varied between the first or secondhalf period of a horizontal scanning period depending upon the level ofthe discrimination signal, and the selection period during which aselection voltage is applied to a scanning line 312 in an odd-numberedrow may be fixed to the second half period of a horizontal scanningperiod.

Furthermore, although in the above-described embodiment, the gray leveldiscrimination circuit 2505 determines the level of the discriminationsignal SG to specify the mode, it is to be understood that the presentinvention is in no way limited to that.

For example, a processing circuit (not shown) which supplies gray dataDn to the C driver 250 may determine the level of the discriminationsignal SG depending upon an execution result of an application program,or a user may specify the discrimination signal SG by operating a switchprovided for that purpose.

In FIG. 1, each TFD 220 is connected to a corresponding data line 212,and each liquid crystal layer 118 is connected to a correspondingscanning line 312. Alternatively, each TFD 220 may be connected to ascanning line 312, and each liquid crystal layer 118 may be connected toa data line 212.

The TFDs 220 used as switching elements in the liquid crystal panel 100may be replaced with another type of two-terminal switching elementssuch as a ZnO (zinc oxide) varister, an MIS (Metal Semi-Insulator)element, a series or parallel connection of two such elements inopposite directions, or a three-terminal element such as a TFT (ThinFilm Transistor) or an insulating gate field effect transistor.

In the case in which TFTs are employed as the switching elements, TFTsmay be formed, for example, by first forming a thin silicon film on thesurface of the device substrate 200, and then forming sources, drains,and channels in the thin silicon film. In the case in which insulatinggate field effect transistors are employed as the switching elements,they may be formed, for example, by employing a semiconductor substrateas the device substrate 200 and forming sources, drains, and channels onthe surface of the semiconductor substrate. In this case, however,because the semiconductor substrate is not transparent to light, thepixel electrodes 234 are formed of metal such as aluminum such that thepixel electrodes 234 serve as reflective electrodes, and thus theresultant display device serves as a reflective type display device.

In the case in which three-terminal elements are used as the switchingelements, not only either the data lines 212 or the scanning lines 312are formed on the device substrate 200, but both the data lines 212 andthe scanning lines 213 are needed on the device substrate 200 such thatthey cross each other on the same device substrate 200. This results inan increase in the risk of forming of a short-circuited path.Furthermore, TFTs are more complicated in structure than TFDs, and thusa more complicated production process is needed to produce TFTs.

The present invention may also be applied to a passive liquid crystaldisplay which uses STN (Super Twisted Nematic) liquid crystal and whichdoes not need switching elements such as TFDs or TFTs. The pixelelectrodes 234 may be formed of reflective metal or an additionalreflecting layer may be formed under the pixel electrodes 234 so thatthe display device serves as a reflective device. Furthermore, thereflecting layer may be formed so as to be very thin so that the displaydevice serves as a transflective device.

In the display device described above, a liquid crystal is used as theelectrooptical material. However, it is to be understood that thepresent invention may also be applied to a display device which displaysan image using an electrooptical effect, such as an electroluminescencedisplay, a fluorescent display tube, or a plasma display. That is, thepresent invention may be applied to any display device having a similarstructure to that described above. Some specific examples of electronicdevices using the above-described display device are described below.

An example of a mobile personal computer using the above-describeddisplay device as its display is described below. FIG. 19 is aperspective view illustrating the structure of the personal computer. InFIG. 19, the personal computer 2200 includes a main part 2204 having akeyboard 2202 and a liquid crystal panel 100 serving as a display.Although a backlight device is disposed on the back of the liquidcrystal panel 100 to achieve good visibility, it cannot be viewed fromthe outside, and thus it is not shown in FIG. 19.

An example of a portable telephone using the above-described displaydevice as its display is described below. FIG. 20 is a perspective viewillustrating the structure of the portable telephone. In FIG. 20, theportable telephone 2300 includes a plurality of operation controlbuttons 2302, an earpiece 2304, a mouthpiece 2306, and theabove-described liquid crystal panel 100. Also in this portabletelephone, a backlight device is disposed on the back of the liquidcrystal panel 100 to achieve good visibility, it cannot be viewed fromthe outside, and thus it is not shown in FIG. 20.

An example of a digital still camera using the above-described displaydevice as its viewfinder is described below. FIG. 21 is a perspectiveview illustrating the structure of the digital still camera, whereinexternal devices connected to the digital still camera are also shown.

In conventional cameras, a film is exposed to an optical image of asubject. In contrast, in the digital still camera 2400, an optical imageof a subject is converted into an electric signal by an imaging devicesuch as a CCD (Charge Coupled Device) thereby producing an image signal.The digital still camera 2400 includes the above-described liquidcrystal panel 100 disposed on the back of a case 2402 so that an imageis displayed on the liquid crystal panel 100 in accordance with theimage signal output from the CCD and thus the liquid crystal panel 100serves as a viewfinder for displaying an image of a subject. A photosensing unit 2404 including an optical lens and the CCD is disposed onthe front side (back side in FIG. 21) of the case 2402.

When a human operator decides to take a picture displayed on the liquidcrystal panel 100, he/she presses a shutter button 2406. In response,the image signal produced by the CCD at that moment is transferred to amemory on a circuit board 2408 and stored therein. In this digital stillcamera 2400, a video signal output terminal 2412 and a datacommunication input/output terminal 2414 are disposed on a side face ofthe case 2402. As shown in FIG. 21, as required, a television monitor2420 can be connected to the video signal output terminal 2412 and apersonal computer 2430 can be connected to the data communicationinput/output terminal 2414. If a predetermined operation is performed,the image signal stored in the memory on the circuit board 2408 isoutput to the television monitor 2420 or the personal computer 2430.

In addition to the personal computer shown in FIG. 19, the portabletelephone shown in FIG. 20, and the digital still camera shown in FIG.21, the display device according to the present invention may also beused in other various electronic devices such as a liquid crystaltelevision, a video tape recorder with a viewfinder or a monitor, a carnavigation device, a pager, an electronic notepad, a calculator, a wordprocessor, a workstation, a video telephone, a POS terminal, a deviceincluding a touch panel, or the like without departing from the spiritand scope of the present invention.

According to the present invention, as described above, it is possibleto reduce the number of times the voltage levels of signals applied tothe data lines are switched in an operation of displaying a gray-scaleimage. As a result, a reduction in electric power consumed when thevoltage levels are switched can be achieved.

What is claimed is:
 1. A method of driving a display device so as todisplay a gray-scale image by driving pixels disposed at locationscorresponding to respective intersections of a plurality of scanninglines extending along rows and a plurality of data lines extending alongcolumns, said method comprising: selecting a single scanning line fromsaid plurality of scanning lines during one horizontal scanning period;applying a selection voltage to said single scanning line during one ofa half period of said horizontal scanning period; selecting a secondsingle scanning line adjacent to the previously selected single scanningline during a subsequent one horizontal scanning period; applying aselection voltage to said second single scanning line during another oneof the half periods of said one horizontal scanning period;simultaneously applying a turn-on or turn-off voltage to a pixel at alocation corresponding to a selected scanning line via a correspondingdata line such that the turn-on voltage is applied during a period witha length corresponding to a gray level in the period during which saidselection voltage is applied, and the turn-off voltage is applied duringthe remaining period.
 2. The method of driving a display deviceaccording to claim 1, further comprising: specifying whether a modeshould be changed, wherein if said specifying step specifies that themode should be changed, when said second single scanning line isselected in said subsequent one horizontal scanning period, theselection voltage is applied to said second single scanning line duringone of half periods of said horizontal scanning period.
 3. The method ofdriving a display device according to claim 2, wherein when a number ofpixels successively aligned a long a column in which pixels that will bedisplayed in a single color of either black or white exceeds apredetermined number of pixels located along a single scanning line thatwill be selected, said specifying step specifies that the mode should bechanged.
 4. The method of driving a display device according to claim 2,wherein when the number of pixels successively aligned along a column inwhich black and white pixels will be displayed alternately exceeds apredetermined number of pixels located along a single scanning line thatwill be selected, said specifying step specifies that the mode shouldnot be changed.
 5. A driver circuit for driving a display device so asto display a gray-scale image by driving pixels disposed at locationscorresponding to respective intersections of a plurality of scanninglines extending along rows and a plurality of data lines extending alongcolumns, said driver circuit comprising: a scanning line driver circuitthat: selects out of said plurality of scanning lines a single scanningline during one horizontal scanning period, applies a selection voltageto said scanning line during one of two half periods that saidhorizontal scanning period has been divided into, selects a singlescanning line adjacent to the previously selected scanning line during asubsequent horizontal scanning period, and applies a selection voltageto said adjacent scanning line during another of the two half periodsthat said horizontal scanning period has been divided into; and a dataline driver circuit that applies a turn-on or turn-off voltage to apixel at a location corresponding to the single scanning line selectedby said scanning line driver circuit via a corresponding data line, suchthat the turn-on voltage is applied during a period with a lengthcorresponding to a gray level in the period during which said selectionvoltage is applied, and the turn-off voltage is applied during theremaining period.
 6. A display device for displaying a gray-scale imageby driving pixels disposed at locations corresponding to respectiveintersections of a plurality of scanning lines extending along rows anda plurality of data lines extending along columns, said display devicecomprising: a scanning line driver circuit that: selects out of saidplurality of scanning lines a scanning line during one horizontalscanning period, applies a selection voltage to said scanning lineduring one of two half periods that said horizontal scanning period hasbeen divided into, selects a single scanning line adjacent to thepreviously selected scanning line during a subsequent horizontalscanning period, and applies a selection voltage to said adjacentscanning line during another of the two half periods that saidhorizontal scanning period has been divided into; and a data line drivercircuit that applies a turn-on or turn-off voltage to a pixel at alocation corresponding to the single scanning line selected by saidscanning line driver circuit via a corresponding data line, such thatthe turn-on voltage is applied during a period with a lengthcorresponding to a gray level in the period during which said selectionvoltage is applied, and the turn-off voltage is applied during theremaining period.
 7. The display device according to claim 6, whereinsaid pixel includes a switching element and a capacitor driven by saidswitching element.
 8. The display device according to claim 7, whereinsaid switching element is a thin film diode having aconductor/insulation/conductor structure, and one end of said thin filmdiode is connected to either said scanning line or said data line, theother end being connected to said capacitor.
 9. An electronic deviceincluding a display device according to claim 6.